Access networks connect business and residential subscribers to the central offices of service providers, which in turn are connected to metropolitan area networks (MANs) or wide area networks (WANs). Often, these access networks are hybrid fiber coax (HFC) systems with an optical fiber based feeder network between the central office and a remote node, and an electrical distribution network between the remote node and subscribers. In so-called “Fiber-to-the-x” (FTTx) access networks, the copper-based distribution part of the access network is replaced with optical fiber, e.g., fiber-to-the-curb (FTTC) or fiber-to-the-home (FTTH). In doing so, the capacity of the access network is increased.
Passive optical networks (PONs) are optical networks that can be used in such applications. A PON is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises. A PON consists of at least one optical line terminal (OLT) at the service provider's central office and a number of optical network units (ONUs) near end users. A PON configuration can reduce the amount of fiber and central office equipment required compared with point to point architectures. Examples of types of PONs that are defined by international standard bodies include Ethernet Passive Optical Networks (EPONs) and Gigabit Passive Optical Networks (GPONs).
Often an OLT at the central office will have several ports, each of which interfaces with several ONUs. Data received from the ONUs at each of these ports is stored in a buffer in the OLT and queued for transmission through an uplink port to a backplane. The backplane typically connects the OLT to an external network, such as the Internet, a private network, or to other OLTs.
Each one of the ports at the OLT has a maximum bandwidth at which data can be received from the ONUs. This maximum bandwidth is typically enforced by a dynamic bandwidth allocation (DBA) algorithm. However, the uplink port through which the data is transmitted from the OLT to the backplane also has a maximum bandwidth. If the sum of the maximum bandwidths of each of the ports servicing the ONUs is greater than the maximum bandwidth of the uplink port, then congestion may occur in the OLT. For example, if the OLT has two ports, each servicing 256 ONUs, and each port allows a maximum of 10 Gbps of data to be received from the ONUs, then the maximum rate of data received at the OLT is 20 Gbps (2 ports at 10 Gbps each). However, if the uplink port through which data is forwarded from the OLT to the backplane only has a maximum bandwidth of 10 Gbps, then congestion may occur during periods of time when the total rate of data received from all of the ONUs across each of the ports is greater than 10 Gbps.
Like reference numerals are used in different figures to denote similar elements.